1. Field of the Invention
The present invention relates to butt-welded cans, and more particularly it relates to laser butt-welded cans each having a weld portion which is provided with a coating film for preventing melted metal particles from adhering to the weld portion at the time when can blanks of sheet steels are butt-welded, the aforesaid weld portion being maintained at a low carbon concentration, and a process for manufacturing the above-mentioned cans.
2. Description of the Prior Art
Heretofore, to manufacture can by roll-forming thin metal plates and then welding the edges thereof, a resistance seam welding (lap welding) technique has been mainly used. In this weld process, the edges of each plate are lapped over each other, and therefore some through holes are liable to appear in a double seaming portion. Further, since the lapped edges of the metal plate are heated and welded, an unpainted portion called a margin is required as much as a width of at least 2 mm in the joined portion of each blank plate for cans, and the inner surface corresponding to the margin must be coated again for anticorrosion and the outer surface of the can has a large unprinted area disadvantageously.
On the other hand, no less than about 2,000 kinds of lasers have been developed of late since the oscillation of a ruby laser succeeded in 1960, and they are now widely employed in various fields.
In the field of thermal treatments which includes welding, cutting and the like, since a carbonic acid gas laser (CO.sub.2 laser) with continuous oscillation was developed, the practical utilization of the laser has been made.
At present, the CO.sub.2 laser having an output of 20 kW or less is on the market and is tried in the thermal treatments (welding, cutting, surface treatment, padding and the like).
The features of the laser weld are as follows: (1) Welding can be accomplished under noncontact; (2) since the area to be heated is small and a welding velocity is high, the influence of heat on the weld portion is small; (3) the heat working of high-temperature-melting materials, heat-resistant alloys and the like is possible; and (4) the weld between different kinds of metals is also possible. In particular, the CO.sub.2 laser has the following features: (1) A high output up to 20 kW can be produced; (2) continuous oscillation and repeated pulse oscillation can be provided; (3) laser conversion efficiency is high (about 20% or more); (4) propagation loss in the air is small; (5) accuracy requirement for optical elements is not so severe; (6) since the CO.sub.2 laser is a gas laser, there is no problem of the breakage of a laser medium due to a high output; and (7) since a material to be consumed is substantially a gas, maintenance cost is lower as compared with the other kinds of lasers.
If metal cans can be manufactured by the use of the CO.sub.2 laser, the following advantages are obtained which are superior to the conventional manufacturing process by the utilization of resistance heat:
(a) Butt-welding can be realized, and therefore any step is not present in a joint portion, and double seaming of lids can be carried out without any problem. Accordingly, the appearance as well as the performance of the thus manufactured cans is also excellent.
(b) Since weld width is as narrow as about 0.2 mm, the manufactured cans have a good appearance. In the case of the conventional resistance weld process, a margin of about 2 mm is necessary, as described above.
(c) The cans having small diameters such as outer cans for dry cells and cans for Aerosol cans can be manufactured. However, in the resistance welding process, an electrode bar is required to be disposed inside each can, and thus the acceptable diameter of the, cans is limited.
(d) In addition to tinplates, metals such as tin free steel and Al can also be welded.
Even in the cans welded by the use of the CO.sub.2 laser, the butt edges of the metal are melted, and therefore an unprinted margin is required along the edges of the can blank, though it is narrower than in the conventional resistance welding process. In particular, for the inner surfaces of the cans, the so-called repair coat is carried out by applying an organic resin to the weld portion and its vicinity on the inner surfaces in order to ensure anticorrosion after welding.
In order to manufacture the laser-welded cans excellent in anticorrosion and sealing properties, it is necessary that the surface of the weld portion is smooth so as to facilitate the repair coat by the application of an organic resin after welding and is excellent in flanging, double seaming, beading, necked-in working and the like which are needed in manufacturing the cans.
In the conventional technique, however, the margin must be taken along the butt edges of the can blank, and the melted metal particles fly about in welding and then adhere to the weld portion and its vicinity again, so that the surface of the weld portion becomes rough, which leads to the problem that the application of the organic resin for the repair coat cannot be achieved perfectly.
Further, in the conventional process, when a lid is mounted on each can body in the double seaming manner, some cracks take place in the weld portion at times, and for this reason, the laser-welded cans having excellent anticorrosion and sealing properties cannot be manufactured stably.
The above description is concerned with the weld portion on the inner side of the can.
For the treatment of the outer side of the can, it is essential to make the most of energy from a laser beam for welding the metal plate. Needless to say, it is effective for this purpose to take the extensive margin, but it should be considered that also on the outer surface of the can, the melted metal particles fly about, though the amount of the metal particles is less than on the inner side of the can. In this sense, it is preferred that certain measures are taken to prevent the melted metal particles from adhering to the outer surface. Further, the present invention aims at putting the manufacturing process of the welded cans by the CO.sub.2 laser to practical use, and for the purpose of keeping up the advantageous appearance that the weld width is narrow, the can blank is preferably used in which the outer surface is also painted all over, since it is very difficult to provide the margin as narrow as about 0.2 mm.
On the other hand, metals such as iron and aluminum are believed to have a reflectance of 90% or more at an oscillation wavelength in the CO.sub.2 gas laser and vicinities, and when the margin is taken, most of the laser output irradiated on the material is reflected on its surface. In consequence, it is thought that the power, which can be absorbed by the irradiated surface and its vicinity and be then converted into heat, is 10% or less.
Some patent publications and literature have reported that the absorption of the laser beam is heightened by applying a suitable coating film to the metal (Japanese Patent Laid-open Publication Nos. 56-160,893, 56-160,894 and 60-18,291). These publications describe the surface treatment and cutting (grooving) of metals, but embodiments of coating materials for welding are not seen anywhere.
If a suitable coating material is acquired which, however, is not always essential in the manufacture of the cans, the application of the coating material is possible all over the outer surface of the can, so that the margin can be omitted therefrom, the adhesion of the melted metal particles can be prevented perfectly, and a printed area on the outer surface thereof can be enlarged, which is also preferable from the viewpoint of the appearance of the cans.